1. Technical
The present invention relates to a stereo camera unit used for three dimensional measurement, and more particularly, to a stereo camera unit in which multiple image-capturing devices are fixed to a camera stay with predetermined arrangement.
2. Related Art
In a known image processing method as a three dimensional measurement technique using images, has been known, in which a target object is captured by right and left cameras (stereo camera unit) from different positions and correlation between both of the images is derived on the basis of the pair of images captured so that a distance is calculated from parallax of the object. In this kind of image processing, a so-called stereo method is used. In the stereo method, camera parameters set in advance such as a focal point distance and an interval (camera base line length) between light receiving surfaces of image sensors provided in the camera are used to derive the distance between the stereo camera and the target object, based on the principle of triangulation. In the image processing according to the stereo method, successively shifting and overlapping two image signals obtained from the stereo cameras, and the position where the two image signals match each other is derived.
The precision of the distance measured based on the stereo method depends on the precision of the position where the cameras are attached. Therefore, if the positions of the light receiving surface of the cameras are displaced, error occurs between the measurement distance and the actual distance.
Japanese Unexamined Patent Application Publication No. 2009-265412 describes a technique for reducing the error. Specifically, with reference to FIG. 3 and its related description therein, a bottom surface 13a, a right side wall 13c, and a lower side wall 13d of a sensor accommodating unit 13 provided in a camera main body 11 disposed on a camera stay 4 are used as layout reference surfaces for a back surface, a right side surface and a lower side surface of an image sensor 12. Accordingly, the back surface of the camera main body 11 is pressed onto the bottom surface of the sensor accommodating unit 13 by elastic force of the elastic sheet 18, whereby the position thereof is accurately fixed.
However, the cameras used in the technique sometimes cannot be preferably fixed to the camera stay.
Specifically, in the above technique, a relatively large charge coupled device (CCD) sensor is used as an image sensor used for the stereo method. Therefore, urging force of the elastic member is used to bring a particular portion of the sensor itself into contact with another portion of the camera unit, whereby the sensor can be positioned.
However, a complementary metal oxide semiconductor (CMOS) sensor is used as an image-capturing device in place of the CCD in some cases. Even when the CMOS sensor captures an image against the sunlight, smear does not occur in the principle, and therefore, this is suitable for vehicle image sensor. When compared with a CCD sensor, the CMOS sensor is small, and therefore, it is difficult to place the small CMOS sensor at a particular position of the stereo camera unit with urging force of the elastic member.
A solution to this problem is a structure where a small CMOS sensor is mounted on a mount board, and this mount board is fixed to the camera stay. However, in general, the mount board made of glass epoxy and the camera stay made of metal such as aluminum have different thermal expansion rates. Specifically, the thermal expansion rate of the camera stay made of aluminum is 22×10−6/degrees, whereas the thermal expansion rate of the mount board made of glass epoxy is 15×10−6/degrees. Accordingly, the expansion rates of them both due to temperature change are different, and the relative position between the lens fixed to the camera stay and the CMOS sensor mounted on the mount board is changed, and therefore, obtained images are also affected by this deviation. In this case, the position of the epipolar line derived from the image is disturbed. Thus, when a matching point of the right image is searched with the left image according to the stereo method, the matching point cannot be found, and it is impossible to calculate correct distance data.
Still further, when resin material is used for the mount board, creep occurs, and with this creep, even when axial force is applied to the mount board with a bolt, the bolt is loosened because of the thinned mount board, and the position of the image-capturing device mounted on the mount board may be deviated from a predetermined position.